Climate and Atmospheric Science (ICAS) PhD Projects
Development and investigation of a new numerical modelling approach for simulating turbulent atmospheric flows over steep terrain
Supervisors: Dr Alan Gadian, Dr Sarah-Jane Lock and Dr Andrew Ross
The advent of computers in the 1950s generated the first numerical weather prediction (NWP) models. Since then, massive advances in computer technology have allowed NWP models to grow in capacity and complexity, and have resulted in great improvements in the accuracy of weather forecasts. However, some of the numerical methods used to solve the basic dynamical equations in most current NWP models have remained relatively unchanged from those very early models.
Recent developments in computers are allowing researchers to push existing NWP models to the limits of their capabilities by, for example, running the models at very high resolutions. The results are not always more accurate, faster forecasts; sometimes, the push to higher resolutions can generate instabilities in the model solutions, which lead to inaccurate or incomplete forecasts. One such example is in the study of air flow over and around steep and complex terrain – an important phenomenon in weather forecasting, since the presence of hills can strongly influence the development of localized storms and subsequent potential flooding.
It has long been known that the existing methods in many NWP models – of using “terrain-following” grid coordinates (Fig. 1), where the vertical grid levels follow the shape of the underlying topography – struggle to produce accurate and stable solutions for winds around steep or sharply varying topography. The research group at the University of Leeds has developed a model – the Microscale Model – for simulating flows over steep hills that uses a new method for representing the uneven lower boundary – a “terrain-intersecting” grid (Fig. 2), whereby the vertical levels remain horizontal throughout the domain, and are intersected by uneven topography. The Microscale Model has been shown to produce good results for idealized non-turbulent flows over steep hills, and offers a useful research tool for very high-resolution, idealised studies of mountain flows. At a broader level, the method has potential for incorporation into the next generation of NWP models, which look to exploit the latest in numerical model development.
This PhD project will extend the Microscale Model by introducing a large-eddy simulation (LES) scheme to enable simulations of turbulent flows generated by friction at the lower boundary. The model's potential will be explored for more real-world scenarios – including the generation of turbulent eddies from flow over complex terrain and extending to the study of the complex flows that take place in and above hills covered by a forest canopy. The capability of the turbulence model will be demonstrated through numerical experiments of idealised flows, comparison with other NWP models and verification against measurements taken during observational campaigns.
The exposure of instabilities in existing NWP methods combined with very recent advances in computers and computer architecture is driving many leading weather research facilities (e.g. UK Meteorological Office and National Center for Atmospheric Research, US) to invest considerable resources in building the next generation of NWP models, designed to exploit future computers to their fullest. The present interest in the methods that underpin NWP models makes this a particularly exciting time to be involved in numerical model development. By exploring a novel approach for solving flows in a complex fluid dynamics environment, this project will offer further evidence of the potential for new numerical methods for use in future NWP models
The project offers an exciting opportunity to work within a highly experienced interdisciplinary group with extensive experience of numerical model development, atmospheric flow over hills and turbulent flows. The group is part of the internationally renowned Institute for Climate & Atmospheric Science within the School of Earth & Environment. The group has close collaborations with colleagues at the US National Center for Atmospheric Research (NCAR) and the UK Meteorological Office. It is planned that funding would be secured to permit the successful candidate to spend some time working with leading scientists at NCAR, Colorado and the UKMO; with a further possibility of visits to work with partners in Germany.